1. Field of the Invention
This invention relates to the diagnosis of proteoglycan deficiency in articular cartilage based on magnetic resonance images (MRI) of the articular cartilage, and more particularly, based on a quantified signal intensity of each pixel of the magnetic resonance image extending across a depth of the articular cartilage.
2. Description of the Related Art
In magnetic resonance imaging (MRI), which is widely used for diagnostic purposes in medicine, a large magnet supported by complicated electronics and computers is employed for image acquisition. When a patient is made to lie in the magnet's magnetic field, individual atoms within the patient's tissues and organs become aligned with the magnetic field. Radiofrequency pulses are then transmitted at a defined rate to the magnetized atoms in the area of interest to thereby elevate the energy levels in the atoms. During a pause period between pulses, the atoms relax and part of the energy gained becomes released. This process represents the unique phenomenon of magnetic resonance. A receiving antenna collects the released energy signals which are then subjected to image processing to convert the energy signals into light dots (pixels) having an illumination level (e.g. a gray-scale) corresponding to an intensity of the energy signals. Thousands and thousands of such light dots form an image. The thus obtained MR image is displayed on a television monitor, printed as a hard copy image and/or stored on a magnetic tape or other recording medium.
An MR image provides structural information, such as size and shape, regarding most tissues and organs in the body, and permits the visual detection of changes in such structural characteristics. For example, if a person develops a meniscal or anterior cruciate ligament tear as a result of a sports related injury, an MRI of the joint will reveal the presence of a discontinuity, swelling and/or shortening in the image of the torn tissue. The structural image information provided by the MRI helps to decide whether a patient requires surgical repair or other remedial action.
FIG. 1 is a parasagittal section view (lateral to midline) of the human knee joint. Reference numeral 101 denotes the femur; 102 denotes the articularis genus muscle; 103 denotes the quadriceps femoris tendon; 104 denotes 104 denotes the suprapatellar fat body; 105 denotes the suprapatellar synovial bursa; 106 denotes the patella; 107 denotes the subcutaneous prepatellar bursa; 108 denotes the articular cavity; 109 denotes the infrapatellar fat body; 110 denotes the patellar ligament; 111 denotes the synovial membrane; 112 denotes the subcutaneous infrapatellar bursa; 113 denotes the deep (subtendinous) infrapatellar bursa; 114 denotes the lateral meniscus; 115 denotes the tuberosity of tibia; 116 denotes the bursa under lateral head of gastrocnemius muscle; 117 denotes the synovial membrane; 118 denotes the articular cartilages; and 119 denotes the tibia. Articular cartilage covers the opposing femur and tibia bone ends in the human knee joint. The articular cartilage, which is rich in extracellular matrix and poor in cellularity, has shock absorption and lubrication functions based on its visco-elasticity which depends on the high water content of its extracellular matrix. Normal human articular cartilage has a water content of 73%-81% (w/w) on a weight basis. Proteoglycans (PG) are the vital organic component required for the functions of articular cartilage. PG contains numerous sugar chains, namely glycosaminoglycans, which contain negatively charged groups such as carboxylates and sulfate groups. These water absorbing, i.e. hydrophilic, groups attract an excess of water hydrogen atoms and water carrying cations. The wide spread network of PG retains this water in the cartilage matrix. A decrease in PG causes changes in the amount and state of water contained therein, resulting eventually in cartilage dysfunction. Thus a PG depletion is indicative of cartilage degeneration and precedes such problems as osteoarthritis.
Although the MRI is used to visually detect structural changes in the articular cartilage of post-trauma joints, currently no non-invasive diagnostic tool is available to detect a biochemical change such as PG depletion in the articular cartilage at very early stages of cartilage degradation. Detection of PG depletion in cartilage prior to a structural change taking place could be extremely beneficial because steps could be initiated to preserve the cartilage by therapeutic intervention. Once PG depletion has started, the surface of the cartilage begins to break after 1-2 years (fibrillation). As a result of fibrillation, usually in about 5 years the cartilage becomes thinned resulting in a narrowing of the joint space. Therefore any attempt to protect or preserve the cartilage must be made before initiation of surface breaking.
Present techniques employing MRI are not capable of detecting biochemical changes, particularly PG depletion, that develop in articular cartilage following joint trauma several years in advance of structural disorganization referred to as osteoarthritis (OA).